In all kinds of electronic hardware, capacitors are employed to store energy, for example, for stabilizing DC voltage levels or for filtering. A special kind of capacitors are multi-layer ceramic capacitors, which may be surface mounted onto printed circuit boards.
Multi-layer ceramic capacitors may have a higher reliability and may tolerate higher ambient temperatures than electrolytic capacitors and thus may be employed in high demanding applications, such as gate drivers of power converters.
In order to increase the capacitance, a large number of ceramic capacitors may be connected in parallel. The realization of large capacitance by paralleling surface mounted ceramic capacitors on printed circuit boards may be limited by the reliability of the assembly: Large ceramic capacitors tend to develop cracks much easier (e.g. during the manufacturing process and the assembly process), and the larger the number of parallel capacitors, the larger the statistical probability that at least one capacitor has a failure. A single capacitor failing into electrical short-mode is usually sufficient to disable the whole electronic assembly.
However, small cracks in ceramic capacitors may initially not be detected. After a while, for example after delivery, the crack may turn from an open connection into an electrical short resulting in failure. It may be that all parallel capacitors are shorted, the voltage at this point of the circuit may drop to zero, and the whole system comprising the electronic assembly (such as a power converter) may stop working.
For example in US 2009/0086457 A1, a ceramic capacitor is protected from cracking by covering it with a soft resin.
GB 2 487 495 A shows a capacitor bank with several capacitors, which may be disconnected from the capacitor bank with a sense/control circuit. The sense/control circuit 106 comprises a MOSFET for disconnecting the capacitor.
US 2011/069425 A1 shows an array of stacked capacitors, which are grouped into groups that may be disconnected from the array via a switching device. The switching device comprises two p-type FETs connected in parallel with respect to each other and in series with a capacitor. A sensor unit composed of several inverters provides a voltage to the gate of one of the FETs, which voltage depends on the leakage current though the capacitor.